In other words, the "security" of the application is entirely a facade. I know the vulnerability in question is a form of unchecked input parameters, but there's something deeper going on here, since the developer in question was told to make a security layer that couldn't be tampered with, and instead made something that's just a UI that LOOKS like it's secure.
It occurs to me that it would be useful to have a term for this. We already have "security through obscurity", and "security theater" to describe other behaviors people engage in they think is security. Is there any term yet for this? Security Facade is the best I can come up with.